In a continuous ink jet printing system, conductive ink which includes a carrier fluid such as water or an organic solvent and a colorant, such as a dye or pigment is continuously recirculated through the system to a print head. The print head generates a plurality of ink drops which are selectively charged and deflected such that some of the drops fall on a print receiving medium and some of the drops are caught and recirculated. Due to evaporation of the carrier fluid during such recirculation, the colorant concentration in the recirculating ink may increase. It is desirable for proper operation of the ink jet printing system to maintain the colorant concentration in the ink to a desired predetermined range. When printing rates are high, there is very little evaporation of carrier fluid from the ink and the ink concentration can be maintained simply by adding ink from a supplemental ink supply. Alternatively, when the printing rates are low or when the printer is in an idle state with all of the ink recirculating, the evaporation rate of carrier fluid is high. In this case, it is necessary to add carrier fluid without colorant from a replenishment carrier fluid supply to maintain the colorant concentration level within the desired predetermined range for proper operation of the ink jet printing system.
Prior art ink replenishment systems have employed ink viscosity measurement or ink optical density measurement to determine the amount of replenishment carrier fluid to add to the ink to maintain the desired concentration of colorant. The electronics requirements are complex and expensive and calibration can be an inconvenience. The light source and detector(s) in the optical density measurement technique need to be recalibrated and/or changed with different inks. Furthermore, the cells through which the ink flows in the optical density measurement apparatus are quite thin and distortion of the geometry of the cells due to pressure changes in the ink can effect the accuracy of the measurement.
Another prior art system for replenishing ink in an ink jet printer is described in U.S. Pat. No. 4,121,222 issued Oct. 17, 1978 in the name of Diebold, et al. This system uses a printed drop count to determine how much replacement ink should be added and employs a balance scale to determine how much replenishment carrier fluid is needed. This system suffers from the problem that the hardware is complex and expensive. It is difficult to recalibrate when different batches of ink having slightly different concentrations of colorant and carrier fluid are used in the ink jet printer. It further suffers from the drawback that the measurement of ink by counting drops is an indirect measurement of ink volume and not as accurate as directly measuring a physical property of the ink.
There is a need therefore for an improved system for replenishing ink or carrier fluid in an ink jet printer to control ink concentration and maintain the desired ink resistivity that is easily calibrated, inexpensive and capable of maintaining the ink concentration in the ink jet printer during both high and low printing rates.